Some types of gas operated rifles or light machine guns have an automatic firing mechanism which is actuated by the pressure waves created by an ignited cartridge and bullet as the bullet travels down the rifled barrel. The automatic firing mechanism loads the next cartridge to be fired into the breach of the rifle. In such rifles or light machine guns, the pressure waves travel through a gas system connecting the barrel and the loading mechanism. However, when a blank cartridge is fired there is no bullet to create the proper pressure-time relationship needed by the rifle or light machine gun in order to function in an automatic or semi-automatic mode. If the exploding gases from the blank cartridge are permitted to exit the muzzle of the barrel without proper restriction, the pressure-time relationship in the barrel and gas system will not match the energy requirements needed to make the automatic firing mechanism function. The rifle or light machine gun will accordingly not operate automatically when firing blank cartridges. To create the required pressure wave and achieve automatic firing, it is necessary to restrict the escape of gases from the muzzle of the rifle or light machine gun.